This disclosure relates to masking in photolithography.
Photolithography is used to print patterns that define integrated circuits onto semiconductor wafers. Typically, a pattern on a photomask is imaged by a highly accurate imaging system onto a silicon wafer that is coated with photosensitive resist. Continued improvements in photolithography have enabled the printing of ever finer features. This has allowed the integrated circuit (IC) industry to produce more powerful and cost-effective semiconductor devices.
As IC feature sizes drop to the subwavelength range, processes involved in photolithography are facing new and more difficult challenges. For example, subwavelength distortions and defects produced during optical exposure of a photomask can cause degraded circuit performance or even a complete failure. Two emerging technologies, a phase shifting technique and an optical proximity correction (OPC), have enabled the improvements in performance of subwavelength design by reducing defects and fixing distortions.
The phase shifting technique refers to the modulation of projected light at the mask level to improve the resolution and depth of focus. Shifting the phase of the light utilizes more of the optical spectrum than with light intensity attenuation, thus enabling finer geometries. The OPC works by adding features to layouts at the mask level to minimize the effects of optical process distortions. The OPC works effectively in conjunction with the phase shifting technique to resolve distortions that result from the xe2x80x9cproximity effectsxe2x80x9d (distortions caused by nearby objects) as well as the diffusion and loading effects of resist and etch processing.
Defects on at least one photomask are detected by patterning alternating dies on a wafer with different process conditions. The different conditions, such as a length of exposure time and an optical focus condition, are configured to highlight and detect defect areas.